In a semiconductor device, it is effective to use a material having a high critical electric field in order to realize a high breakdown voltage and a low on-resistance. Since a nitride semiconductor has a high intensity of the critical electric field, the semiconductor device realizing the high breakdown voltage and the low on-resistance is achieved by using the nitride semiconductor.
In the nitride semiconductor device, a gate insulator on a hetero-junction, a barrier on the gate insulator, and a gate conductor on the barrier are formed, and characteristics of the nitride semiconductor can thus be drawn out.